Production of caustic and chlorine by electrolysis of brine is well known in the art. The electrolysis is carried out in an electrolytic cell which consists in general of an anode, a cathode, an anode compartment and a cathode compartment. In one of the more recent types of such an electrolytic cell, the two compartments are separated from one another by a fluorine-containing cation exchange membrane.
Such an electrolytic cell can be operated more efficiently and economically as the current efficiency is increased, and as the operating voltage is lowered. Inasmuch as very large quantities of caustic and chlorine are produced by electrolysis of brine daily, even a very small improvement in the current efficiency or operating voltage of chloralkali cells will lead to saving of large amounts of money and conservation of large amounts of energy.
The operating voltage of a chloralkali cell is made up of a number of component parts, of which one is that part of the voltage drop at the cathode known as the hydrogen overvoltage. A lowering of the hydrogen overvoltage will result in lowering of the overall cell voltage and consequently make the process more economical.
In U.S. Pat. No. 4,105,516, a method for electrolyzing brine with an iron cathode at a lowered cell voltage is disclosed wherein an electrolytic hydrogen evolution catalyzing transition metal is added to the catholyte liquor while carrying out the electrolysis. One example is to add a solution of ferrous chloride to the catholyte compartment. However, the effect is not lasting, and to maintain the effect the transition metal must be repeatedly added at intervals ranging from 7 or 10 days down to as often as 6 hours. It is therefore necessary to provide for separating the transition metal which contaminates the product caustic.
In U.S. Pat. No. 4,160,704, a method for reducing the cathodic hydrogen overvoltage potential in a chloralkali cell is disclosed wherein a solution of metal ions selected from a group which includes iron, cobalt and others, all in the form of cations, is introduced into the catholyte solution, and a current is passed to plate the metal ions onto the cathode. In this method, the solution of combined metal cations was fed into the cathode compartment for many (20) hours, or was recirculated through the cathode compartment over a period of one hour; in such procedures, however, the ferrous cations added would precipitate as insoluble ferrous hydroxide when there is any caustic in the cathode compartment unless a complexing agent such as tartrate is also present. Alternatively, an iron element was placed in the cathode compartment, or a cathode chamber fabricated of stainless steel was employed; in the latter case after 27 days of operation, the overpotential of the cathode had been lowered, but the cathode chamber itself was corroded.
In U.S. Pat. No. 4,298,447, a cathode which lowers hydrogen overvoltage in chloralkali electrolysis is disclosed which has particles consisting essentially of crystals of alpha-iron adherent to the surface of a cathode substrate; the method disclosed is the placing of a cathode substrate, a specified electrolyte and particles comprising grains of alpha-iron in an electrical apparatus, and passing an electrical current; the electrical apparatus can be a membrane-type chloralkali cell. However, it has now been found that this method has the disadvantages that (1) a portion of the iron particles added to the cathode compartment is discharged from the cell along with the caustic product, (2) some of the iron particles form a sludge in the bottom of the cathode compartment, and (3) some of the iron particles have, at times, adhered to the ion-exchange membrane and caused iron deposits in the membrane which, in sufficient amount, can cause delamination of the membrane and an increase in the voltage.
It is therefore an object of this invention to provide an improved method for making an iron-activated cathode for a chloralkali cell.
It is another object of this invention to provide a method for lowering the hydrogen overvoltage of a chloralkali cell.